Why Does Array.prototype.push Return the Length and Not the Appended Item?

The Curious Case of Array.prototype.push: Why It Returns Length Instead of Something More Convenient

Array.prototype.push is a JavaScript method that extends an array with one or more elements. Unlike its more intuitive counterparts that return the newly added item or the modified array itself, push curiously returns the array's new length. This unconventional behavior has raised questions about the design decisions that led to its current implementation.

Historical Context

To uncover the rationale behind this design choice, we delve into the history of push. As recounted by TC39's communication hub, push debuted in JavaScript 1.2 (Netscape 4) in 1997, inspired by Perl's array manipulation functions. Initially, push returned the last pushed item, adhering to Perl 4 conventions. However, in JavaScript 1.3 (Netscape 4.06), push shifted its behavior to align with Perl 5, returning the array's new length.

Rationalization

Why was returning the new length deemed more appropriate than returning the appended item or the mutated array? This question, unfortunately, remains unanswered. The historical record provides scant details on the reasoning behind this deviation from Perl 4. However, we can speculate on some potential considerations:

• Simplifying Implementation: Returning the new length may have simplified the implementation, requiring less code to track and manipulate the added item or the modified array.
• Consistency: Returning the new length aligns with other methods that manipulate array lengths, such as splice and shift. This consistency may have been prioritized for a more intuitive and cohesive interface.

Alternative Options

Had the designers not opted for returning the new length, other options could have been considered:

• Return the New Item: This would provide direct access to the added element, potentially saving developers the need to manually retrieve it using the new length.
• Return the Modified Array: Returning the array itself, as modified by the push operation, could eliminate the need for an additional step to gather the updated array.

Conclusion

Although the specific rationale behind Array.prototype.push returning the new length is not entirely clear, its historical origins in Perl and the potential advantages of simplicity and consistency may have influenced its implementation. While this design decision may not align perfectly with developer expectations, it provides a well-defined and consistent behavior for manipulating arrays in JavaScript.

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